Discord slash-command schema definitions for the Lemon adapter.

Pure data: the @*_command maps describe each application command exactly as registered with Discord via Nostrum.Api.ApplicationCommand. Extracted from LemonChannels.Adapters.Discord.Transport so the transport GenServer holds orchestration only; the transport re-exports slash_commands/0 and the *_command_schema/0 accessors as thin delegations to preserve its public API.